Understanding the Impact of the Skin Microbiome on Dermatological Assessments and Therapeutic Innovation

Abstract

The human skin microbiome, defined as a multifaceted ecosystem comprising bacteria, fungi, viruses, and mites, plays a pivotal role in maintaining skin homeostasis and regulating immune responses. In recent years, an increasing amount of evidence has illuminated the considerable influence exerted by microbiomes on the pathophysiology of dermatological ailments. This review provides a comprehensive synthesis of contemporary findings concerning the microbiome’s role in acne, aging, hyperpigmentation, and hair disorders, while also addressing the emerging concept of the gut–skin axis and how it could interfere in these skin disorders. Alterations in microbial composition, referred to as dysbiosis, have been associated with inflammatory processes and barrier dysfunction, thereby contributing to the severity and chronicity of diseases. Distinct microbial profiles have been identified as correlating with specific skin conditions. For instance, variations in Cutibacterium acnes phylotypes have been associated with the development of acne, whereas alterations in Corynebacterium and Staphylococcus species have been linked to the processes of aging and pigmentation patterns. Furthermore, the composition of the microbiome is examined in relation to its impact on cosmetic outcomes. It also engages with increasing interest in the modulation of microbiota through the topical application of bioactive compounds. The incorporation of prebiotics, probiotics, and postbiotics into cosmetic formulations constitutes a novel strategy aimed at enhancing skin health. In the domain of dermatological therapies, postbiotics have emerged as a significant class of substances, particularly due to their remarkable stability, safety, and immunomodulatory properties. These characteristics position them as promising candidates for incorporation into dermatological treatments. Recent studies have underscored the significance of microbiome-informed strategies within the domains of therapeutic and preventive dermatology, emphasizing the potential of such approaches to positively influence patient outcomes. As our understanding of this field continues to evolve, skin microbiomes are poised to emerge as a pivotal area of focus in the realm of personalized skin care and treatment. This development presents novel and innovative approaches for the management of skin conditions, characterized by enhanced specificity and efficacy.

1. Introduction

The microbiome refers to the set of microorganisms and their physiological and biochemical interactions with the environment, which create distinct ecological niches [1]. Since the invention of the microscope in the 17th century, it has been possible to observe microorganisms in the human body. The biggest milestone in microbiome research was the Human Microbiome Project (HMP), which ran from 2007 to 2016.

The HMP involved sequencing around 3000 reference bacterial genomes. This resulted in the world’s largest metagenomic database on bacterial, fungal, viral, and protist community composition. This database made it possible to integrate metagenomic, transcription, protein, and metabolite profiles of the human microbiome [2,3,4]. The initial phase of the HMP focused on investigating microbial communities in healthy individuals, prioritizing the nasal, oral, skin, gastrointestinal, and urogenital regions. The second phase, Integrative HMP (iHMP), examined various omics data from the microbiome and host [4,5].

The interaction between microorganisms and humans is intrinsic and has likely existed since the beginning of human existence. Currently, much of the available information focuses on the relationship between bacteria and humans, largely due to limitations in analytical methods [6].

The human microbiome varies significantly in composition across different body sites, each of which harbors distinct microbial communities adapted to the local environment [7]. Skin microbiota consists of diverse microorganisms, including bacteria, fungi, and viruses. These organisms live in a commensal relationship and play important roles in defending the immune system, producing substances, and protecting against pathogens [8].

The composition of this microbiota is affected by various factors, including age, gender, environment, lifestyle, sebum, and hygiene habits [9]. The skin can be divided into three main categories: sebaceous regions (e.g., the face, chest, and back); moist regions (e.g., the inside of the elbows, the backs of the knees, and the groin); and dry regions (e.g., the forearms and palms) [10].

The microbial composition varies between individuals and between different areas on the same person. Thus, variations in bacterial groups in the skin microbiome depend on area type—wet, dry, or oily—and this diversity is widely studied [1].

In general, bacteria from four main phyla can inhabit the skin: Actinobacteria (including Corynebacterium, Propionibacterium, Cutibacterium, Micrococcus, Actinomyces, and Brevibacterium); Firmicutes (including Staphylococcus, Streptococcus, and Finegoldia); Proteobacteria (including Paracoccus and Haemophilus); and Bacteroidetes (including Prevotella, Porphyromonas, and Chryseobacterium) [11]. The abundance and balance between these phyla vary according to the skin region (moist, dry, oily), influencing local immune responses and skin conditions [12,13].

In synergy, these bacteria contribute to immune modulation and skin barrier function, with Actinobacteria and Firmicutes often promoting homeostasis and defence against pathogens such as Cutibacterium acnes and Staphylococcus epidermidis, which can modulate immune receptors (e.g., TLR-2). In contrast, Proteobacteria and Bacteroidetes influence inflammation and microbial diversity [13,14].

Regarding fungi, common genera on the skin include Aspergillus, Malassezia, Penicillium, Candida, and Cryptococcus, with Malassezia (M. globosa, M. restrita, and M. sympodialis) being the most prevalent in various conditions [11,15,16]. Additionally, skin mites, including species such as Demodex folliculorum and D. brevis, coexist in this habitat. These mites are considered permanent ectoparasites of the skin microbiome [17]. Information on viruses is limited since their genomes are small, making metagenomic detection difficult [18]. However, viruses from Polyomaviridae, Papillomaviridae, Circoviridae, Caudovirales, Adenoviridae, Anelloviridae, and Herpesviridae families are frequently identified. Their interactions with the microbiome and skin, however, still require further studies [16].

Understanding the interactions between microorganisms and the skin requires grasping the concepts of symbiosis and dysbiosis [19]. In symbiosis, microorganisms coexist with the host and often help protect against pathogens. For instance, the strain of Staphylococcus aureus promotes the growth of Staphylococcus epidermidis, benefiting the host. Dysbiosis, on the other hand, occurs when there is an imbalance in the skin’s microbial composition, either due to the colonization of pathogenic microorganisms or abnormal growth of bacterial strains in relation to other commensals. Dysbiosis can lead to clinical conditions such as acne, psoriasis, atopic dermatitis, and rosacea, which can negatively affect an individual’s quality of life [10,20,21].

2. Intestine-Skin Axis

Of the diverse microbiota observed, the intestinal microbiota is the most abundant, containing approximately 100 trillion microorganisms [22].

The skin and intestine share several similarities: both are covered by epithelial cells and have a large surface area (25 and 30 m², respectively); both have rich vascular and nervous structures that come into contact with the external environment; and both have a high rate of cell renewal and a large number of microbial cells (10¹² and 10¹⁴, respectively) [15,23]. In addition to these similarities, the intestinal microbiome is a dynamic system in which changes occur gradually over the years, causing a decline in diversity. Finally, both microbiomes act as barriers and must be maintained in homeostasis [15,24]. A diagram (Figure 1) showing the similarities between the intestine and the skin highlighted the barrier function performed by both microbiotas. Interference with the intestinal microbiome can lead to dermatological conditions, exposing that the immune system can be modulated through the microbiome, thereby interconnecting the skin and intestinal microbiomes.

The gut microbiome influences several distant organs and tissues, such as the brain (gut–brain axis) and lungs (gut-lung axis). The skin is not excluded from this influence; its relationship with the gut is called the gut-skin axis [15,26]. This bidirectional pathway between the skin and intestinal microbiomes is currently mediated by the immune and endocrine systems [15,23,27].

Although the mechanism has not yet been elucidated, several studies show a relationship between the gut microbiome and skin conditions. For example, gastrointestinal disorders are present in skin diseases, and vice versa [23,26].

Since 70–80% of immune cells are in the intestine, there is a complex interaction between intestinal microbiota, the intestinal epithelial layer, and the local mucosal immune system [28]. The gut microbiome participates in regulating at least 30 hormone-like compounds, such as short-chain fatty acids (SCFAs), secondary bile acids, cortisol, and neurotransmitters [29]. Thus, it acts in the regulation of inflammatory processes. Microbial communities primarily maintain intestinal barrier integrity by degrading complex, indigestible polysaccharides into vitamins (specifically vitamin K and B12) and SCFAs (specifically butyrate and propionate). SCFAs can modulate the immune system, which may be beneficial in preventing, modulating, or improving the prognosis of dermatological disorders resulting from immune system dysregulation [30].

One theory proposes a relationship between dysbiosis and increased intestinal permeability. In this theory, metabolic byproducts produced by bacteria enter the bloodstream and accumulate in the skin. This decreases keratin synthesis and epidermal differentiation, which impairs the skin barrier and increases the risk of inflammation [26,27].

In symbiosis, the microbiota produces metabolites, neurotransmitters, and hormones that enter circulation and influence the skin. Dietary elements can also access the skin directly or through processing by microbiota. Additionally, the skin produces chemicals that impact on the gut, such as vitamin D [31].

In dysbiosis, toxins cross the compromised intestinal barrier and allow bacteria to enter the bloodstream. The liver usually captures commensal intestinal bacteria and bacterial products/components, preventing systemic inflammation. Inadequate processing of these substances by the liver favors a pro-inflammatory environment that can manifest skin problems [32].

For many years, the predominant methodologies employed for the analysis and identification of microorganisms were predicated on morphological identification, physiological characteristics, and biochemical traits. Fortunately, the extensive genetic profiling of diverse microorganisms and increasingly robust databases has led to significant advances in this field [33].

The advent of molecular techniques for expeditious microbial identification has signified a substantial advancement across a plethora of scientific disciplines, including biotechnology, the food industry, diagnostic medicine and genetic engineering, to name but a few. Despite the existence of common phenotypic traits among these microorganisms, genotypic variations have now become a widely employed tool for their identification according to their taxonomic groups [34,35].

Omics tools, including metagenomics, proteomics, lipidomics, transcriptomics and metabolomics, are high-throughput techniques that characterise and quantify biological molecules. These techniques elucidate the structure, function and dynamics of an entire organism or organisms [5,36].

3. The Influence of Microbiome on the Skin

3.1. Acne

Acne vulgaris (acne) is a chronic inflammatory skin disease that results in the formation of skin lesions and adverse psychological and social effects in patients, thereby diminishing their quality of life and productivity [37,38,39]. It is among the most prevalent dermatological conditions and affects a significant proportion of the global population, with 85% of individuals experiencing acne at least once during their lifetime. This statistic positions acne as the eighth most common disease [40].

Using keywords such as human skin microbiome, cosmetics and microbiome, symbiosis, probiotics and prebiotics, the most cited articles are presented in

Table 1. The most cited articles related to acne.

Author/Title	Summary and Conclusion
SCHNEIDER et al., 2023	Puberty influences facial skin microbiome changes, highlighting shifts in microbial diversity and C. acnes strain composition associated with increased sebum production and acne development. Puberty significantly alters the skin microbiome, and a distinct acne-associated microbiome emerges in late puberty, suggesting potential targets for acne treatment.
SCHOLZ; KILIAN, 2016	Analyzes of whole-genome sequences of Propionibacteriaceae, revealing that traditional classification does not reflect genetic relationships. It proposes splitting the genus Propionibacterium into three new genera—Acidipropionibacterium, Cutibacterium, and Pseudopropionibacterium—and redefines species boundaries, especially for skin-associated bacteria like P. acnes. The paper concludes that genome-based taxonomy clarifies evolutionary relationships, leading to a more accurate classification system that distinguishes skin-specific bacteria from those in other environments, improving understanding of their adaptation and biology.
ZAENGLEIN, 2018	Clinical features, pathogenesis, psychological impact, and management strategies for acne vulgaris, emphasizing a multifactorial cause involving sebum production, follicular hyperkeratinization, inflammation, and bacteria Effective treatment requires combination therapy targeting different pathogenic mechanisms, with an understanding of individual patient factors and potential side effects to improve outcomes and quality of life.
DAGNELIE et al., 2019	How severe back acne and mild-to-moderate facial acne are is linked to alterations in skin bacterial communities, notably reductions in diversity and imbalances in Propionibacteriaceae, Staphylococcaceae, and Enterococcaceae families, highlighting microbiota’s role in skin inflammation. Skin inflammation in acne correlates with decreased bacterial diversity and specific microbial imbalances, suggesting potential for treatments that modulate skin microbiota to restore balance and reduce inflammation.
DRÉNO et al., 2020	Acne development is linked to a loss of diversity among skin bacteria, especially different strains of Cutibacterium acnes and Staphylococcus epidermidis. This imbalance triggers immune responses and inflammation. It emphasizes that targeting microbiomes for treatment could be a promising alternative to antibiotics. Restoring microbial diversity and balance may lead to more effective, tailored, and eco-friendly acne therapies, moving beyond traditional antibiotic use.
FITZ-GIBBON et al., 2013	Compares C. acnes strains on acne patients versus healthy skin, revealing that while bacterial abundance is similar, strain types differ significantly—some strains are linked with acne, others with health. Genetic analysis suggests these differences may influence the bacteria’s role in disease. Strain-level variation in C. acnes is crucial in acne development; understanding these differences can guide targeted therapies and improve skin health strategies.
WEI et al., 2022	Microbial diversity across facial sites (forehead, cheek, nose) in Chinese and American populations using shotgun metagenomics. It finds distinct microbial compositions and functions at different sites, with the nose showing higher levels of porphyrin-producing bacteria like C. acnes, linked to inflammation. The patterns suggest a combination of neutral drift and niche selection influences these biogeographic differences. Facial microbiomes exhibit site-specific variability influenced by both stochastic and deterministic processes, and these patterns differ between populations, impacting skin health and disease susceptibility.
BARNARD et al., 2020	Porphyrin production among skin bacteria, highlighting that C. acnes type I strains produce significantly more porphyrin, especially those linked to acne. Vitamin B12 increases porphyrin levels in acne-associated strains but not in healthy skin strains. The composition and strain diversity of skin bacteria influence porphyrin levels and inflammation. The skin microbiome’s species and strain makeup determine its metabolic activity and inflammatory potential, providing insights for developing acne treatments that modulate bacterial composition and porphyrin production.
GRICE; SEGRE, 2011 [8]	Diverse microbial communities living on the skin, their roles in protection and immune education, and how various environmental, genetic, and physiological factors influence this ecosystem. It emphasizes the importance of under-standing these interactions for skin health and disease. Maintaining the delicate balance of skin microbiota is crucial; disruptions may lead to skin disorders, and understanding these interactions could guide future therapies and microbiome-based treatments.
MARPLES; DOWNING; KLIGMAN, 1971	How C. acnes bacteria contribute to the production of free fatty acids (FFA) on human scalp skin by breaking down surface lipids, and how antibiotics that suppress C. acnes reduce FFA levels. C. acnes is a primary source of lipolytic enzymes that generate FFAs, which are implicated in conditions like acne; reducing C. acnes levels decreases FFA production, highlighting its key role in lipid metabolism on skin.

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Acne is characterized by elevated sebum production, presence of hyperkeratinized follicles, and colonization of the follicles by Cutibacterium acnes [41,42,43]. The spectrum of acne lesions encompasses open and closed comedones, inflammatory papules, pustules, and cysts, culminating in painful nodules. In some cases, acne can result in permanently disfiguring scars [40].

The treatment of acne is contingent upon its severity. For mild to moderate forms, treatment typically involves application of topical medications that possess both antibiotic and anti-inflammatory properties. Examples of such medications include topical retinoids, hormonal antiandrogens, benzoyl peroxide, and topical clindamycin, among others [44,45,46].

In cases of more severity, the administration of retinoids and oral antibiotics is recommended. However, it is important to note that the long-term administration of these medications has been shown to increase the risk of microbial resistance to antibiotics. There is an increasing prevalence of reported resistance to topical macrolides in more than 50% of C. acnes strains across numerous countries, thereby diminishing their effectiveness. This phenomenon is compounded by the safety concerns associated with their potential adverse effects [47,48,49].

While the gastrointestinal microbiome is one of numerous factors contributing to acne, in the case of acne vulgaris, it exerts a discernible influence on skin conditions [50]. The prevailing hypothesis suggests that a Western diet, characterized by low fiber and high fat content, induces fundamental alterations in the composition of the gut microbiota, which, in turn, has been shown to be associated with the development of metabolic and inflammatory skin diseases [43].

The presence of microorganisms in the human gut has been shown to play a significant role in the pathogenesis of acne, primarily by modifying the mTOR (mammalian target of rapamycin) pathway and increasing intestinal barrier permeability, once again linked to a diet, characterized by a high consumption of foods with a complex mixture of fat, high glycemic index, and dairy products contributing factors to the exacerbation of acne. This is attributed to the elevated insulin levels that result from such diets, which have been observed to lead to an increase in androgenic parameters, such as insulin-like growth factor-1 (IGF-1). These parameters have been associated with not only the worsening of existing acne lesions but also the emergence of new lesions [43,50,51].

Cutibacterium acnes is a commensal bacterium that inhabits the sebaceous regions of the skin. C. acnes possess genes that encode lipases capable of degrading lipids. These lipases facilitate the hydrolysis of triglycerides, resulting in the release of fatty acids into the skin. There, C. acnes aggregate on these acids and colonize the sebaceous glands [52]. The expertise in this symbiotic relationship is that the same fatty acids that the bacteria use are also capable of contributing to the acidification of the skin’s surface pH, which inhibits common pathogens known as Staphylococcus aureus and Streptococcus pyogenes, which are unable to proliferate in acidic pH [8,53,54].

It has been established that certain species of Cutibacterium, including Cutibacterium acnes, Cutibacterium avidum, Cutibacterium granulosum, and Cutibacterium namnetense, are distinguished by their capacity to synthesize porphyrin. These bacteria are more prevalent in the posterior region of the nose compared to other anatomical regions of the face [55]. Porphyrins function as metabolites resulting from inflammatory processes and play a key role in the synthesis of vitamin B12 [56,57].

Despite their importance in the production of essential molecules, these substances have pro-inflammatory properties, and high levels of them have been linked to acne [6,56]. Furthermore, studies have demonstrated that some young people with delayed puberty exhibit a metabolic signature that is distinct from that of their peers. This signature is characterized by an increase in the metabolism of porphyrin, histidine, and propanoate [57].

Recent studies in metagenomics have indicated a variation in the strain structure of C. acnes between healthy individuals and those suffering from acne, although both groups have a similar relative abundance [58]. In individuals diagnosed with acne, there is a higher prevalence of phylotype IA1 (84.4%) compared to those classified as healthy. This strain has also been found in greater quantities on the backs of patients suffering from acne [59].

However, in contrast to the prevalence of the IA1 phylotype of C. acnes on the face, a reduction in its prevalence is associated with a greater severity of acne on the back. A correlation may exist between the composition of the microbiota and the severity of the condition, as more severe symptoms have been associated with greater alpha diversity and a higher presence of Gram-negative bacteria, including Faecalibacterium, Klebsiella, Odoribacter, and Bacteroides. In individuals with mild acne, these variations are not observed [60].

Research findings suggest that acne may emerge due to an imbalance between S. epidermidis and C. acnes, as both species interact and play pivotal roles in maintaining skin homeostasis [52]. S. epidermidis has been shown to regulate the proliferation and inflammation caused by C. acnes. The anti-inflammatory effects of S. epidermidis are attributed to lipoteichoic acid, which is the result of the fermentation of glycerol and the production of succinic acid. In contrast, C. acnes, which inhabits the pilosebaceous follicles, acts by inhibiting the growth of S. epidermidis by preserving the acidic pH of the follicle, promoting the hydrolysis of sebaceous triglycerides and the secretion of propionic acid [59].

Consequently, it can be posited that the emergence of acne is not driven by uncontrolled proliferation of C. acnes, but rather by a disruption in the equilibrium between its distinct phylotypes, accompanied by dysbiosis of the microbiome. Restorative interventions aimed at enhancing microbiome diversity have been demonstrated to attenuate inflammatory responses.

3.2. Aging

Skin aging, both extrinsic and intrinsic, is associated with several molecular mechanisms. Reactive oxygen species (ROS) have been demonstrated to play a key role in the process of cellular aging. The presence of these molecules in the antioxidant balance is of significance. At low levels, they act as signalling molecules; however, at high levels, they have the potential to cause stress and even DNA damage, thereby promoting mutations and inducing cell death, both internally and externally [54,55]. While these ROS have the capacity to activate immune system cells, an overabundance of ROS leads to damage to vital macromolecules, such as lipids and proteins. The restoration of antioxidant balance can be achieved through the introduction of external antioxidants or the augmentation of the activity of antioxidant genes present in the skin [54].

As part of the discussion related to aging, a bibliographic search was conducted using the descriptors “microbiome,” “skin,” “wrinkles,” “probiotics,” “microbiota,” “aging,” “ageing,” “facial,” “anti-aging,” and “human” were used. These terms were searched in various combinations, and studies were then selected based on the abstracts presented. In addition to studies in English, literature in Portuguese and Spanish was also selected. Further research was also conducted in textbooks and on the research platform to determine and detail concepts and definitions. Among the literature selected after screening (over 173 studies), only 97 were used. The main ones are described in

Table 2. The most cited articles related to aging.

Author/Title	Summary and Conclusion
MCLOUGHLIN et al., 2022	Relationship between the skin and its microbiome, focusing on microbial dysbiosis and probiotic interventions for its management. The topical use of probiotics and post-biotics is promising but still lacks clinical evidence and a clearer understanding of the mechanisms of skin dysbiosis.
RATANAPOKASATIT et al., 2022	Influence of human microbiomes on skin aging and interventions, such as probiotics, to modulate health. The interactome is a promising future strategy for slowing down ageing, but clinical studies are lacking.
SFRISO et al., 2020	Overview of the skin microbiome, sampling and analysis techniques, and skin care strategies to restore and balance the microbiota. Advances in DNA sequencing and extraction techniques have broadened knowledge of skin microbiome.
ISHAQ et al., 2021	Review linking intestinal dysbiosis, ageing and oxidative stress, highlighting the role of Lactobacillus strains in modulating these processes. Lactobacillus shows promising effects in modulating aging and intestinal health, but robust clinical trials to confirm doses, strains and efficacy in humans are lacking.
RUSSELL-GOLDMAN; MURPHY, 2020	Skin aging and its implications for the body’s well-being. Skin ageing is influenced by multiple external factors, brought together in the concept of the exposome. These factors affect the epigenome and compromise skin cell integrity.
HABEEBUDDIN et al., 2022	Applications and influences of topical probiotics on skin health and diseases, and their relationship with skin microbiome. Recent advances highlight the role of topical probiotics in the treatment of inflammatory skin conditions associated with dysbiosis. Clinical trials investigate their efficacy and safety in various skin conditions.
MAGUIRE; MAGUIRE, 2017	Importance of skin microbiome, emphasizing the role of prebiotics and probiotics in skin modulation, balance and health. Modulation of the microbiome with prebiotics and probiotics, combined with stem cells, can treat conditions such as acne, ageing and Epidermolysis bullosa (EB).
BOYAJIAN et al., 2021	Impact of microbiome on aging, highlighting cellular senescence, the gut-skin axis and the potential of probiotics and prebiotics as anti-senescent therapies. The interaction between the microbiome, cellular senescence and senescence-associated secretory phenotype (SASP) directly affects the health of the skin and the body. Probiotics and prebiotics are promising as anti-senescent therapies.
WOOLERY-LLOYD et al., 2022	Influence of skin microbiome on facial aging, addressing the use of oral and topical prebiotics, probiotics and postbiotics to reduce the signs of aging. Oral and topical probiotics, prebiotics and postbiotics can reduce fine lines, signs of ageing and improve hydration by balancing the skin’s microbiome.
BONTÉ et al., 2019	How intrinsic and extrinsic factors—such as aging, UV exposure, pollution, and lifestyle—impact skin structure and function. It emphasizes the role of oxidative stress, hormonal changes, and cellular senescence in skin ageing. It also explores how these changes impair wound healing in the elderly and reviews strategies to mitigate skin ageing through skincare and medical interventions. Maintaining skin health requires understanding and addressing factors like oxidative damage and hormonal alterations. Improving healing in aged skin is crucial, and skincare strategies targeting these mechanisms can help slow ageing and enhance skin repair.

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Exposure to ultraviolet (UV) radiation increases ROS formation, compromising the body’s ability to neutralize them and triggering inflammatory processes that hinder collagen synthesis [61,62]. As we age, metalloproteinases (MMPs) are negatively regulated, resulting in collagen degradation and reduced skin elasticity [63]. Thus, the balance between collagen production and degradation is disrupted, causing wrinkles and other visible signs of aging [62,63].

Changes in intestinal and skin microbiota are suggested to be related to skin aging and may contribute to wrinkles, one of the strongest characteristics of aging. Suppressing the SASP (Senescence-Associated Secretory Phenotype) pathway, which is activated by the accumulation of senescent cells in the skin, prevents the positive regulation of MMPs [26,64]. In other words, modulation of the intestinal microbiome may be an interesting approach to prevent skin aging [64].

As previously mentioned, microbiomes act as a barrier and contribute to health. To maintain homeostasis and stability, a mutualistic relationship must exist between the microbiota and the host. This allows the skin to remain healthy [65,66].

An altered microbiome can cause skin aging, which can then alter its composition [26,67]. Despite these possible changes, this relationship can favor “anti-aging” because a balanced microbiota without dysbiosis provides additional protection against premature aging [68].

In addition to providing a first line of defense against invading pathogens and boosting skin immunity by positively regulating the secretion of defensive biomarkers, the microbiome can offer UV protection (Staphylococcus aureus converts histidine into urocanic acid, which protects the skin against radiation) and antineoplastic activity (Staphylococcus epidermidis) [69].

Physical changes resulting from aging are one of the main causes of skin microbiome dysbiosis and are directly linked to the microbiome. For instance, changes in pH, sebaceous secretion, hydration, and flattening of the dermo-epidermal junction can alter the microbiota’s composition [64,67,70].

The dermo-epidermal junction strengthens the skin’s structure and facilitates the exchange of nutrients and oxygen between the epidermis and dermis. As one ages, the junction flattens, which weakens the structure, reduces nutrient transfer, and compromises the mutualistic relationship with skin microorganisms. This will favour dysbiosis and weaken defence against pathogens [70].

The decrease in Cutibacterium (and commensal bacteria) caused by decreased sebum production favors the colonization of opportunistic species since decreased sebum production reduces the production of antimicrobial peptides that inhibit the growth and colonization of pathogenic microorganisms [71].

Additionally, studies have correlated two Corynebacterium taxa with wrinkles and age spots (independent of extrinsic factors) [72,73].

3.3. Hyperpigmentation

The production of melanin by melanocytes, as well as the quantity, quality, and distribution of melanin, is fundamental to determining skin, eye, and hair color. Pigmentation disorders can occur due to a reduction (hypopigmentation) or increase (hyperpigmentation) in melanin [68,69].

In order to better understand the mechanisms involved in hyperpigmentation, a survey of articles, monographs, dissertations, theses, and books was conducted using the keywords “skin,” “microbiome,” and “hyperpigmentation” to collect general and specific data on the topic.

Table 3. The most cited articles related to hyperpigmentation.

Author/Title	Summary
ZANCHETTA, C. et al., 2022	Identifies specific skin bacteria associated with hyperpigmented spots (HPS). It reveals that skin microbiota composition and interactions differ between skin with high and low HPS levels, suggesting bacteria may influence dark spot development through immune regulation. The skin microbiota could be a new target for skincare, as it plays a role in the emergence of dark spots and skin health.
WANG, Z. et al., 2017	Demonstrates that the skin microbiome promotes mast cell maturation by stimulating keratinocytes to produce stem cell factor (SCF), involving microbial components like lipoteichoic acid (LTA). Absence of microbiota results in immature mast cells and reduced immune defense, which can be reversed by microbiome restoration. The skin microbiota signals mast cell recruitment and maturation through keratinocyte-derived SCF, revealing a novel mechanism that has implications for skin diseases such as atopic dermatitis.
BOSVELD, C. J. et al.2023	Reviews how mast cells interact with the skin microbiome to maintain skin health, recognizing that this crosstalk influences immune responses and skin diseases like psoriasis and atopic dermatitis. Understanding mast cell-microbiota interactions is crucial for developing new strategies to promote skin health and treat related disorders.
DRÉNO, B; 2019	Skin microbiome’s composition, its importance in maintaining skin health, factors influencing it (such as birth method and environment), and its potential as a target for new dermatological treatments within the framework of ecobiology. The skin microbiome is a vital, dynamic organ that plays a key role in skin health and immune regulation. Restoring and maintaining its balance may lead to innovative treatments in dermatology, emphasizing the preservation of this complex ecosystem.
HE, G. et al. 2017	Explores how fibroblasts from mastitis bovine mammary glands secrete SDF-1, which promotes inflammation and EMT in epithelial cells, contributing to mastitis and early fibrosis. Mastitis-associated fibroblasts produce SDF-1 that drives inflammation and EMT, potentially leading to tissue spread of disease and fibrosis; inhibiting SDF-1 signaling may help mitigate these effects.
You et al., 2013	Investigates how lipoteichoic acid (LTA) from Lactobacillus sakei can prevent UVA-induced skin aging by blocking MMP-1 production and MAPK signaling pathways in human skin cells. It highlights LTA’s potential to act as an anti-photoaging agent by modulating immune responses and inhibiting molecular signals triggered by UVA exposure. LTA from Lactobacillus sakei effectively suppresses UVA-induced MMP-1 expression and associated signaling pathways, indicating its promising role as a natural anti-photoaging agent.
KIM, H. M. et al.; 2014	Demonstrates that L. plantarum HY7714 protects skin cells and mice from UVB-induced aging by reducing collagen-degrading enzymes, inhibiting inflammation-related pathways, and decreasing wrinkle formation. L. plantarum HY7714 shows potential as an anti-photoaging agent, offering a promising strategy for skin protection against UV damage.
GAO, T. et al. 2023	Discusses how probiotics can improve skin health by modulating the gut-skin axis, reducing inflammation, oxidative stress, and supporting immune functions, thus addressing issues like aging, dryness, and skin diseases. Probiotics offer a promising, safe alternative for skin care by maintaining microbiota balance and enhancing skin health through gut-skin interactions, providing a theoretical foundation for future applications.
KAO, H.-J. et al.; 2021	Demonstrates that propionic acid (PA), a metabolite from C. acnes fermentation, inhibits UVB-induced melanin production by suppressing tyrosinase activity via FFAR2, without affecting melanocyte proliferation or disrupting skin microbiota, suggesting a natural, safe skin-lightening approach. PA effectively reduces skin pigmentation caused by UV exposure through natural mechanisms, making it a promising safe alternative for hyperpigmentation treatment.
Hossain et al., 2021	Explores how various stimulus such as UV exposure, allergens, pathogens, and physical injuries—trigger inflammatory responses that influence skin pigmentation. It highlights the role of cytokines and immune cells in regulating melanocyte activity and melanin production, emphasizing the connection between inflammation and pigmentary disorders. Understanding the inflammatory mechanisms affecting melanogenesis can lead to the development of targeted therapies for pigmentation-related skin conditions, emphasizing the importance of controlling inflammatory responses to manage skin pigmentation effectively.

lists some of the most noteworthy, selected works.

The main factors associated with hyperpigmentation are multifactorial and may include sun exposure, dermatological conditions, hormones, age, heredity, skin lesions, inflammation, and acne [74].

Wrinkles and hyperpigmentation also result from the formation of reactive oxygen species, which induce the expression of matrix metalloproteinases (MMPs). MMPs have various impacts on the skin and can lead to premature skin aging [61].

Melanin plays an essential role in protecting against UV radiation and oxidative stress. Several inflammatory cytokines are produced through acute or chronic inflammatory responses. These cytokines modulate the proliferation and differentiation of melanocytes and inhibit gene expression related to melanogenesis, the process by which melanin pigments are synthesized and distributed [75,76].

To investigate whether hyperpigmentation is linked to microbiota, one study found an association between aging and an increase in pigmentation spots, though no significant correlation was identified with other analyzed biometric parameters, such as pH and hydration. However, significant differences in bacterial genera were identified depending on hyperpigmentation intensity [77].

For instance, the genus Eikenella was present at high levels in hyperpigmented skin. The species Eikenella corrodens may regulate melanin production by preventing its degradation. Conversely, the genus Kocuria is often found in skin with less hyperpigmentation because it helps prevent chronic inflammation and skin infections, two main causes of pigmentation spots [77].

Mast cells promote the growth of melanocytes and melanoma cells. Skin with melasma has significantly higher numbers of dermal mast cells than not lesioned or perilesional skin, suggesting that mast cells may be involved in epidermal pigmentation induced by histamine in response to UV radiation [78].

Lipoteichoic acid, produced by S. epidermidis, acts through Toll-like receptor 2 (TLR2) in keratinocytes. This induces the production of stem cell factors, which triggers the recruitment of mast cells in the dermis. It also conditions their function and location [79].

Dysregulation of C. acnes in relation to microbiota can cause post-inflammatory hyperpigmentation (PIH), increased melanin production, and progressive macular hypomelanosis. In PIH, C. acne causes an inflammatory reaction that leads to the secretion of cytokines and other inflammatory mediators by keratinocytes. When these agents bind to Toll-like receptors (TLRs), they cause melanocytes to increase melanin production [80].

The interaction between skin microbiome and UV radiation is complex. While several studies point out the negative impacts of UV rays on skin health, others show that when balanced, microbiomes can offer benefits such as anti-inflammatory and antioxidant properties, as well as protection against UV-induced damage, which is beneficial in preventing hyperpigmentation [81,82].

Studies investigating the effects of UV-B radiation in the absence of microorganisms have proven that microbiota influence UV-B-induced immune responses in the skin. It has also been reported that S. epidermidis and C. acnes can prevent melanocytes from transforming into tumor cells. Large quantities of the genus Kocuria (formerly Micrococcus) were noted on skin with less hyperpigmentation, suggesting that strains such as Kocuria rhizophila (formerly Micrococcus luteus) have antioxidant and UV-protective properties [77,81,82,83].

3.4. Infrared Radiation

The sun emits different kinds of energy, including cosmic rays, gamma rays, X-rays, UVB and UVA radiation, visible light, and infrared radiation [84]. It is widely recognized that light exerts a broad range of effects across various biological domains and has been used as a therapeutic agent for many years [85]. Visible light also includes blue light, which is emitted at wavelengths between 400 and 500 nm. The primary source of blue light is sunlight, but it is also emitted by digital screens—such as those of mobile phones, computers, laptops, and TVs—as well as by light-emitting diodes (LEDs) and fluorescent lighting. Therefore, exposure to blue light is considered inevitable [86].

According to the study conducted by ICHIHASHI et al., 2003 [87,88], approximately 6% of the solar spectrum consists of ultraviolet radiation (UVR) with wavelengths between 200 nm and 400 nm, another 52% consists of visible light (wavelengths between 400 nm and 760 nm), and the remaining 42% is infrared radiation (wavelengths between 760 nm and 10⁶ nm).

Exposure to sunlight can also cause damage to human skin, leading to the formation of free radicals, which are partly responsible for erythema/edema, inflammation, photoaging, and skin disorders, as previously described. However, the skin is also exposed to infrared (IR) radiation, which can generate free radicals in the skin that, depending on the dose, can trigger a cascade of different signaling pathways, inducing either therapeutic or pathological effects. The free radicals formed by infrared radiation may account for up to one-quarter of the amount generated by UVB/UVA exposure [89].

A compendium of articles was compiled associating the terms infrared radiation, microbiome, and human skin. As there were not enough studies available for data extraction and analysis, the terms light, visible light, and ultraviolet radiation were also included at

Table 4. Summary of selected articles related to infrared radiation.

	Articles	Outcomes and Conclusions
1	POLEFKA et al., 2012	Importance of understanding the effects of solar radiation, including infrared, on human skin. Insights into the mechanisms involved in skin responses to solar radiation support the development of effective protective strategies and the prevention of skin damage. Furthermore, the analysis of infrared impact suggests a potential influence on skin health and balance, encouraging the consideration of infrared radiation in therapeutic and cosmetic approaches for maintaining skin health.
2	HAREL et al., 2022	The results provide evidence of the impact of solar radiation, including infrared, on skin microbiome across different populations. Understanding these effects is crucial for developing personalized skincare approaches that consider individual differences and specific exposure to these rays. Identifying infrared-related alterations in the skin microbiome may open new avenues for targeted therapeutic and cosmetic interventions aimed at preserving skin health and function.
3	PATRA et al., 2018	Emphasizes the complex interplay between ultraviolet radiation, skin microbiome, and local immune responses. Understanding these interactions is essential for developing more effective protective and therapeutic strategies. Considering infrared radiation as a relevant component in this context offers a more comprehensive view of the effects of solar radiation on skin health. These findings support the pursuit of personalized preventive and therapeutic approaches that take into account the influence of infrared radiation on the skin microbiome and immune responses.
4	FERNÁNDEZ et al., 2016	Emphasizes the importance of protective approaches against infrared radiation. The use of β-carotene-incorporated bicosomes proved promising in reducing damage induced by infrared exposure. These findings support the development of topical and therapeutic formulations designed to protect the skin from the harmful effects of infrared radiation, thereby maintaining skin integrity and health. Incorporating β-carotene into bicosomes offers an effective strategy to minimize oxidative stress and other damage caused by infrared exposure, presenting a potential solution for preserving skin health.
5	BAROLET et al., 2016	The literature review on the effects of infrared radiation on human skin and the skin microbiome reveals a range of conflicting findings. While infrared radiation may have beneficial effects such as stimulating collagen production, it can also cause cellular damage and premature skin aging at high doses. Therefore, careful consideration of the dose and duration of infrared exposure is essential to avoid adverse effects. These findings underscore the need for a personalized approach to infrared radiation protection, taking into account individual skin characteristics and specific exposure levels.

, where the most related articles were described.

Approximately 40% of the solar radiation reaching sea level is composed of infrared (IR) light (760–1 mm). The shorter-wavelength photons within the near-infrared (NIR) range (NIR or IR-A: 760–1400 nm) can penetrate the epidermis, dermis, and subcutaneous tissue, exerting various biological effects. Both in vitro and in vivo studies have shown that another subdivision of infrared radiation, known as far-infrared (FIR, 3–25 nm), can stimulate cells and tissues [90].

Infrared radiation can penetrate the epidermis, dermis, and subcutaneous tissue to varying degrees, depending on the specific wavelength range. IR exposure is perceived as heat. Free radicals can be triggered by IR radiation and play a significant role in skin physiology. IR exposure can cause oxidative damage, leading to oxidative stress and the generation of free radicals [90]. On the other hand, protection against oxidative stress and free radical-induced skin damage is of critical importance [91].

Exposure to infrared (IR) radiation is an increasing concern due to rising levels of solar exposure and the widespread use of electronic devices that emit IR radiation. Several studies have investigated the effects of IR radiation on the skin and skin microbiome, although findings remain divergent.

Polefka et al. [87] examined the effects of solar radiation on the skin and reported that IR exposure can induce the production of reactive oxygen species (ROS) and oxidative stress, leading to DNA damage and premature skin aging. However, the study did not specifically address the skin microbiome. Although IR radiation is less frequently discussed compared to ultraviolet (UV) radiation, research indicates that it also –contributes to skin damage. IR radiation penetrates deeper into the skin and generates oxidative stress, potentially harming cells and cutaneous tissues. Thus, protective strategies should consider not only UV radiation but also IR exposure to preserve skin health. Another relevant point is the interaction between UV and IR radiation. The authors suggest that simultaneous exposure to both types of radiation may produce synergistic effects, intensifying skin damage. IR radiation may amplify the harmful impact of UV radiation by increasing oxidative stress and cutaneous inflammation. Consequently, protection against both UV and IR radiation is essential to maintain skin integrity and overall health.

Harel et al. [92] investigated the effect of solar radiation on the skin microbiome in two distinct populations: lifeguards who had been chronically exposed to high levels of solar radiation over the years, and ultra-Orthodox individuals who remained protected from sunlight year-round due to heavy clothing. The study found that exposure to infrared radiation led to alterations in the composition and diversity of the skin microbiome, including a reduction in certain bacterial species. These findings suggest that infrared radiation may negatively influence skin microbiome health, as a decrease in beneficial bacteria and an increase in pathogenic species were specifically observed.

Fernández et al. [89] investigated protective strategies against the harmful effects of infrared (IR) radiation through the incorporation of β-carotene into bicosomes—phospholipid-based nanostructures composed of spherical vesicles (approximately 150–250 nm) and discoidal structures (15–25 nm). Their findings demonstrated that this approach effectively reduced IR-induced skin damage and helped preserve skin microbiome. The lipid molecules constituting bicosomes played a key role in neutralizing free radicals and absorbing IR radiation at various wavelengths. These results suggest that lipid-based delivery systems, such as bicosomes, could be harnessed in the development of targeted therapeutic interventions to mitigate IR-induced damage.

Barolet et al. [90] also explored the interaction between infrared radiation and the skin, highlighting both its potential benefits and adverse effects. The authors examined the mechanisms of IR action, including direct tissue heating and the activation of intracellular signaling pathways. A central point of the study was IR radiation’s ability to penetrate deeper into the skin than UV radiation, reaching connective tissues and blood vessels. This deeper penetration has been associated with increased blood circulation and collagen synthesis, potentially improving skin health and appearance. However, the study emphasizes the need for a balanced understanding of IR exposure, considering both its therapeutic potential and its capacity to cause oxidative stress and tissue damage at higher intensities.

Serrage et al. [93] also highlighted that the resident skin microbiome possesses the ability to detect and respond to blue light through expression of chromophores. This can modulate physiological responses, ranging from cytotoxicity to proliferation, and present evidence of the diverse blue light-sensitive chromophores expressed by members of the skin microbiome.

In summary, the influence of infrared radiation and blue light on the human skin microbiome is a developing field of research. The results obtained to date highlight the importance of protecting the skin from infrared radiation and the need for personalized approaches to skin care and its microbiome.

3.5. Hair Disorders

In recent years, scalp has been the focus of several investigative studies that seek to explore the relationship between organisms present in the human host and how these can influence scalp and hair follicle disorders due to microbiota imbalance. The scalp offers a diverse microenvironment for organisms, based on the physiological conditions of its host, including pH, humidity, sebum content and topography [78,79]. During the coexistence process, exchanges occur between the scalp surface and the formed microbiome, in which microorganisms can establish biofilms that will act in a symbiotic, commensal or pathogenic manner [78]. A brief bibliographic survey identified articles related to scalp disorders characterized in

Table 5. The most cited articles related to hair disorders.

Author/Title	Summary and Conclusion
Park et al., 2017	Cohort study to investigate the microbiota of 102 individuals in Korea. Staphylococcus sp. and Malassezia restricta associated with higher incidence in unhealthy skin. Propionibacterium sp. and Malassezia globosa linked to normal scalp.
Saxena et al., 2018	To examine the bacterial and fungal diversity of the scalp microbiome in 140 Indian women. Cutibacterium acnes and Staphylococcus epidermis were reported as the main bacterial species, the former being associated with healthy scalp and the latter, the opposite.
Park et al., 2012	Investigation of the fungal biota present in cases of dandruff and how they affect the scalp, leading to other disorders. Basidiomycota (Filobasidium ssp.) was commonly associated with dandruff, while Ascomycota (Acremonium ssp.), with healthy scalps.
Filaire et al., 2020	Comparative study of 24 healthy men with androgenetic alopecia (AAG) and possible use of Lindera to treat the disorder. Treatment with Lindera extract for 83 days was able to restore the scalp microbiota of patients with AGA, obtaining results similar to those found in healthy individuals.
Wang et al., 2022	Conducted with 95 patients of Chinese origin, the study seeks to correlate the bacterial community in healthy and dandruff scalps. Microbial networks were less integrated in cases of dandruff when compared to the healthy surface. Cutibacterium, Staphylococcus and Malassezia were the most abundant microorganisms on the diseased scalp.
Yu et al., 2021	Evaluation of the microbiota of patients with seborrheic dermatitis and treatment with rhT beta 4 as a promising therapeutic strategy in the prevention and treatment of SD. Compared to treatment with ketoconazole, treatment with rhTβ4 showed a significant improvement in leather homeostasis. Malassezia and Staphylococcus increased in cases of SD.
Grimshaw et al., 2019	Investigative study of the microbiota of Chinese patients with and without dandruff. With a taxonomic study of the main genera found. The most abundant bacterial genera were Cutibacterium and Staphylococcus, while the most abundant fungal genera were Malassezia (M. resctricta with the main alteration).
Hiruma et al., 2014	Investigation of Malassezia fungal genotypes in patients with dandruff and seborrheic dermatitis. Malassezia level is 3× higher in individuals with dandruff, with M. globosa and M. restricta predominating.
Pinto et al., 2019	Investigation of bacterial communities in individuals with alopecia areata. Bacterial distribution with increased levels of C. acnes and decreased levels of Staphylococcus epidermis in patients with AA.
Viduthalai Rasheedkhan et al., 2024	Detailed investigation of the scalp and hair follicles, in healthy individuals and those with dandruff/seborrheic dermatitis. They demonstrated that the microbiome inhabiting hair follicles serves as a reservoir for the scalp microbiome and propionic acid, produced by C. acnes, plays a main role in maintaining microbiome balance

.

As Figure 2 shows, the structure of the hair follicle allows microorganisms with different characteristics to grow. For instance, the follicular cavity enables anaerobic species to grow inside, while aerobic species grow on its surface. Sebaceous and sweat glands in the follicle’s structure also serve as energy sources for some microorganisms. This diversity of microorganisms in a follicle provides protection against pathogens when in balance.

Among the main microorganisms found in the biological environment of the scalp region, Malassezia and Cutibacterium acnes stand out. The Malassezia genus consists of lipophilic yeast species, the vast majority of which are recognized as lipid-dependent, due to their lack of ability to synthesize C14 and C16 fatty acids [96]. For this reason, these organisms require a host that is competent in providing this source of nutrients. Therefore, the human host becomes ideal in this sense.

Cutibacterium acnes is known to produce propionic acids, which can maintain an acidic pH, creating an environment that is not very habitable for the growth of other pathogens [94,97]. However, the human host is complex and, in certain circumstances, such as in cases of microbiota imbalance, interaction with these microorganisms can lead to the appearance of dermatological conditions that can negatively affect the individual [98].

Hair follicle inflammation processes are observed in multiple existing hair diseases. According to Polak-Witka [99], abnormal immune responses are believed to occur when the host microbiota is altered. The microbiome in the region can interact with the host’s keratinocytes and the immune system, consequently generating an increase in antimicrobial peptides, cytokines, chemokines and free fatty acids, bringing benefits or harm to the host [100].

Dandruff and seborrheic dermatitis are common diseases that affect human scalp, differing only in the severity with which they affect the host. They are characterized in 4 phases, which are: (1) Fungi and bacteria interact with the epidermis of the region, (2) Inflammation occurs with progression to clinical symptoms, (3) The proliferation and differentiation phases in the scalp are interrupted and (4) The functional barrier of the skin is paralyzed [93,100].

Seborrheic dermatitis (SD) can be considered a chronic, recurrent inflammatory disease. Characterized mainly by symptoms of itching, redness and scaling of the skin, mainly affecting the scalp, ears and chest. It is suggested that SD has multiple combined factors that lead to pathogenesis [101]. On the other hand, dandruff and seborrheic dermatitis are considered diseases frequently found on the scalp, it is believed that both have the same pathogenesis condition, differing only in the issue of intensity [102].

Unlike SD, dandruff is a non-inflammatory condition, also characterized by flaking and a much milder form of the dermatitis mentioned above [103]. Mainly linked to microbial issues, it is also suggested that it may be caused by external factors, such as excessive exposure to sunlight, irritation from excessive use of shampoo and other cosmetic products, exposure to dirt, dust and others [104].

Androgenetic alopecia (AGA) is another disease that affects many people around the world. It is characterized by the progressive loss of hair strands, due to the reduction in the phase responsible for hair growth (anagen) and the miniaturization of the hair follicle over time. Although it is linked to hereditary function and the age factor, its origin may also be influenced by microorganisms in the region due to the cause of microinflammations [105].

The second most common hair loss disease is alopecia areata (AA), a disorder that affects not only the scalp but also the epidermis of the body in its non-scarring form. In AA, the catagen phase may not occur or may occur very quickly, leading to the occurrence of irregular lesions and lack of hair in the area [106]. Because it has a specific microbiome, it is possible to relate the phases of hair growth to the conditions of the scalp [107]. In addition to the most discussed conditions, other hair diseases can be found and described in literature, such as folliculitis decalvans (FD), scalp psoriasis (PS) and others. Considering that the hair area is always in continuous contact with the external environment and microorganisms, the action of the immune system in the area is necessary. Therefore, hair follicles are considered privileged immunological sites, as they express important immune cells in fighting possible infection [108].

4. Microbiome and the Cosmetics Market

The human skin microbiome became a focus in the first two decades of the 21st century and the publication of studies continues to grow, with the aim of better elucidating the association of microorganisms with skin diseases. The change in perception about microorganisms, going from villains to allies, broadens this context. The cosmetics industry began to explore this relationship, including probiotics and prebiotics in its products to strengthen skin health.

In the initial studies conducted in parallel with the Human Genome Project, when talking about cosmetics, the association was that these products in general, be they soaps, hygiene products or moisturizers, had the potential to alter microbiota in some way, but it was not yet known which [8].

Before discoveries about the human skin microbiome, microorganisms were considered “villains” since there was no understanding that a layer of them is frequently and regularly present under the entire length of human skin. It was believed that they were transient “germs” that, if habits such as washing hands were not adopted, could cause diseases and infections. The latter is true; however, this type of knowledge directly influenced the cosmetics industry, to the extent that one of the most common claims in communication for body soaps for bathing, or the so-called hand-cleaning soaps, had claims on their labels such as “kills/eliminates 99.9% of germs/bacteria” or similar communications.

Over the years and discoveries about beneficial microorganisms, the communication of cosmetic products began to adopt discoveries from the academic environment. In addition to eliminating microorganisms, the current trend is to use them or fragments of them through the use of probiotics and prebiotics in cosmetic formulations. It is easy to understand that there are harmful microorganisms, but maintaining the beneficial ones and commensals on the skin can help eliminate the harmful ones, avoiding discomfort caused by the growth of harmful microorganisms, such as dysbiosis or other skin infections, has become a point of interest. Some cosmetics, such as the thermal waters of La-Roche Posay, France, contain in their formulation Vitreoscilla filiformis, a Gram-negative bacterium classically used for dermatological treatments and associated in some studies with beneficial effects for seborrheic dermatitis and atopic eczema, although the mechanisms for such beneficial effects are not yet fully known [109,110].

The last example concerns treatment using facial cosmetics. However, in Brazil, some brands have explored the use of prebiotics in body lotions, such as the Brazilian brand Natura, which uses prebiotics in its Natura Tododia^® line and promises to “increase resistance against various external aggressors such as microorganisms that can cause diseases and reinforce the skin barrier, increasing protection against environmental variations” [111]. In addition, another brand that has appropriated this communication with operations in Brazil is the American company Neutrogena of the Johnsons & Johnsons group, which in 2023 began to communicate the action of its prebiotic oats in its advertisements, using the concept of what a prebiotic is, in short, it is the food that will serve as a substrate for microorganisms, in this case, resident in the skin. A point of differentiation is that this product is intended for dry, extra dry and sensitive skin [112].

Cosmetic products for the face have sometimes even in-depth explanations on their websites about which prebiotic/probiotic is being used in the formulation. This is the case of the Amilia Repair Multi-Repair Prebiotic Lotion, which contains the prebiotic Alpha-Glucan that “helps maintain and restore the skin’s microbiota, reinforcing the body’s natural defenses.” This claim clearly elucidates the findings of the academic community that by maintaining beneficial bacteria on our skin, they outnumber the harmful bacteria that cause dysbiosis and other infections [112]. On the other hand, there are products that do not specify which prebiotics are being used, such as the Prebiotic Moisturizing Mist, which has the claim “with prebiotic actives that maintain natural defenses and ensure the balance of the skin’s microflora.” The benefit is in line with what prebiotic should do to the skin, but communication presents prebiotics as active ingredients, which can result in a generalization of which prebiotic is present in the formulation [113]. The Sephora Collection facial mask provides another example by communicating as a combination of prebiotics, but without making it clear which prebiotic is used in its formulation, or in this case, which ones.

BioKinder presents the product Probiotic Moisturizing Lotion, aimed at babies. Unlike most of the products covered in this review, it presents the probiotic as a “lysed probiotic”, that is, it is understood that this probiotic is inactivated and only its fragments would be a postbiotic. Although it does not communicate which probiotic is in the formulation, the product includes in its formula of ingredients available for consultation on its website [114]. Lactobacillus Ferment Filtrate (Lactobacillus in its lysed form).

Oceane’s Probiotic Night Facial Serum is one of the few products that state in its communication that the probiotics in its formulation can prevent dermatitis, that is, this is an example of a product that associates its benefits with the prevention of dysbiosis such as dermatitis [115]. Simple Organic’s Prebiotic Gel also claims to prevent dysbiosis, but the example in this case is the prevention of acne [116]. Another interesting product in terms of claims is Vitamédica Bio Serum Probiotic HOF, which contains the probiotic in its formulation in an encapsulated form and communicates which probiotic it is, which in this case is Lactobacillus acidophillus [117].

It is understood that some products covered in this review did in fact present consumer research studies to obtain differentiated claims for the use of prebiotics and probiotics, as is the case of Biossance Squalane + Probiotic Moisturizing Gel, which combines the encapsulation of probiotics with the combination of natural alginate polymers, two main benefits: the first is that it promises to keep the probiotics alive until the moment of application because they are encapsulated and the second links the sensory maintenance of the peak freshness and potency of this raw material [118]. On the other hand, some products seem to surf the high of these ingredients in communication and do not go into detail about what the pre and probiotics in the formulation are, in addition to communicating together with generalist words such as “combination/actives/of prebiotics/probiotics”.

5. Future Perspectives and Conclusions

Human skin plays a crucial role in maintaining homeostasis because it is made up of several layers, each with distinct functions. An imbalance can occur when these layers are affected by internal or external factors, leading to dermatological problems such as acne, hyperpigmentation, seborrheic dermatitis, hair loss, and premature aging.

The gut-skin axis is a mutual interaction between the microbial communities of both sites that communicate through signaling pathways. An imbalance in gut microbiota can lead to skin health issues.

Innovations in metagenomics have broadened our understanding of microbiome diversity and its consequences for skin health. However, understanding the human microbiome remains extremely complicated because the fundamental basis has not yet been fully clarified or standardized. As it has taken several decades for some discoveries about the gut microbiome to be consolidated and new revelations are still emerging, studies on the skin microbiome are just beginning. This directly affects the standardization of this microbiome research, influencing the interpretation of results and the consistency of conclusions.

Future research must focus on longitudinal and interventional studies to determine causal relationships and verify the therapeutic efficacy of microbiome modulation approaches. Additionally, standardizing the sampling, sequencing, and analysis of microbiome-related data is essential to advancing its clinical application.

Postbiotics are gaining popularity in skin care products because they are considered safe and effective agents for treating various conditions. Unlike live probiotics, postbiotics have a more stable formulation and a lower risk of microbial contamination, making them ideal for inclusion in cosmetics. The bioactive compounds in these products can strengthen the skin barrier, regulate inflammation, and restore microbial balance.

In short, recognizing that the human skin is home to a vast community of microorganisms that coexist not only superficially but also in an integrated manner is undoubtedly one of the first steps toward understanding the importance of deepening and harmonizing concepts and results. This ensures that the cosmetic products used daily maximize their effects and benefits.